Soil Perc Testing

Soil testing

Soil testing is used to determine the site and soil suitability for a Private Onsite Wastewater Treatment System (POWTS). The soil test will help determine the type, size, location, and design parameters of the proposed POWTS for both new construction and for replacement systems.  For some reason there is a common misconception that all replacements of failing systems must be mound systems or that from a certain time forward, all new systems have to be mound systems, but this is absolutely not the case.

Soil conditions can differ and vary dramatically from lot to lot, some requiring mounds, some conventional (subsurface) systems, as well as several other types of systems that fall somewhere between these two.

What determines the type of system required on each lot or parcel of land are the results of a “Soil and Site Evaluation” by a Certified Soil Tester / Morphological Evaluator. This evaluation is commonly referred to as just a “soil test”, and just as commonly (although mistakenly) called a “perc test”.

Actual “perc tests” which used to be the common method for determining the design of “POWTS”, was the  method where water was literally poured into holes in the ground for the purpose of measuring just how far into the ground a certain amount of water would go in a certain amount of time , thus determining the soil’s ‘percolation rate’. The ‘perc’  method is no longer the accepted method of soil testing, as the information gathered was far too limited for making accurate determinations of important soil characteristics. Sure, you knew how fast the water went into the ground, but the ‘perc’ method did not indicate just how far this water would travel before it would encounter a “limiting factor”.

A “limiting factor” would be high bedrock, or high groundwater. The “perc rate” may indicate how fast the water is moving, but if the water immediately (within inches or just a few feet) encounters high bedrock or high groundwater, the wastewater would not be treated properly, and very likely contaminate the groundwater, move much more quickly to the groundwater (untreated) through cracks and crevices in the bedrock, and cause a  system to fail prematurely due to less than adequate depths to a limiting factor.

Wisconsin Administrative Codes mandate that any type of system must be designed so that the “bottom” of a soil absorption system will be a minimum of 3 feet above high groundwater or bedrock. Groundwater in this instance does not necessarily refer to the actual water table your well is drilled to, but by code definition could mean any “zones of soil saturation which include: perched water tables, shallow regional groundwater tables or aquifers, or zones that are seasonally, periodically, or permanently saturated.”     This is the answer to many peoples’ dilemma of “Why do I need a mound if my soil is all sand?- It should be a conventional!” Well, even if someone’s soil is all sand, but the soil test shows indications of high groundwater at only 2 feet below the ground from existing grade, there is only one way to design a system which will be 3 feet above the groundwater, and that is by artificially creating a soil absorption system one foot above the ground- and that is what a mound system is- a system designed with the bottom of it’s soil absorption area above the existing grade, to allow for the three foot vertical separation to the limiting factor on sites that have a limiting factor two feet below  grade.

Most soil tests are performed using a backhoe to dig “pits” large enough for the soil tester, (and in most cases an inspector), to closely observe (and hand texture) each “horizon”   ( or layer) within the soil profile. This is done to both observe for the limiting factors discussed above, and to determine several other characteristics of the soil which will determine the type, depth, and square footage of system required on this particular lot.


Mottling /  Redoximorphic features

 When a soil tester is observing the soil profile trying to determine if the “limiting factor” of groundwater is present, the soil tester does not have to see actual water. If the soil tester observes “mottling” (or redox features) in the soil, it is at the depth the mottling is observed that is considered the limiting factor of high groundwater ( with a few  exceptions). Mottling is a discoloration , or “staining” of a color which is not part of the dominant soil color which is an indication that groundwater had, at some point in time, fluctuated up to that elevation and remained there long enough to cause the chemical reaction – or oxidation- to occur to create the mottling. The soil tester must assume that because water had fluctuated that high at some point in the past, that the potential exists that groundwater may fluctuate up to that elevation again at some point in the future. The groundwater elevation is now assumed to be the elevation at which mottling was observed, whether water is present there at the time of the soil test or not.

This above information on “mottling” answers the continuing question asked- “Why do I need a mound-I didn’t see any groundwater?” Actual water does not have to be observed, the mottling is the indication that groundwater has ‘fluctuated’ that high and that there is a potential for the groundwater to rise that high again, and the system must be designed based according to the soil tester’s observations.

If you have specific questions on soil tests or would like to schedule a soil test contact us at  262-968-2550